It is known that the EDM capability of selectively removing surface projections makes the process highly suitable for efficiently machining the parting line of interfitting or mating members, e.g. tools, dies and molds, to remove excess material which impedes a close fit therebetween as required. However, the earlier efforts to successfully utilize the EDM process for these applications have often proved to be unsatisfactory and have left much to be desired. Difficulties especially arise where the interfitting members are large or the surfaces to be interfitted are large in area so that a commercially available or standard EDM machine is inappropriate. Then, not only must a mechanical structure be provided which is adequate to carry the heavy members that may each have a weight of tons and to move them for necessary positioning operations, but a fine servo adjustment is necessary to accurately maintain their relative position. It has been believed to be indispensible to have a high-capacity fluid supply and handling unit including a large worktank necessary to accommodate the large members dipped in the EDM liquid medium as well as high-rate pump, filter and fluid circulating conduits. High operating costs would be required, inter alia, because of the large volume of the EDM liquid medium required to fill and circulate through the large tank. The EDM liquid is indispensable as a medium for sustaining electrical discharges, to assure prompt gap recovery, to cool and carry away EDM products and debris, and to cool the members heated by the electrical discharges.